1. Field of the Invention
The present invention relates to a resonant element used as an angular velocity sensor, acceleration sensor, filter, or the like, and to a vibration adjustment method therefor.
2. Description of the Problems Leading to the Present Invention
FIG. 9A is a perspective view showing a previous resonant element invented by the inventor of the present invention. FIG. 9B is a sectional view taken along the line Ixe2x80x94I in FIG. 9A.
The resonant element shown in FIGS. 9A and 9B is a resonant element 1 which is a microelement produced utilizing a conventional silicon micromachining technique and the like. The resonant element 1 is produced by forming a nitride film 3 on a silicon fixed substrate 2, then forming a polysilicon film 4 thereover, and forming these films 3 and 4 into a predetermined set pattern by dry etching or the like.
As shown in FIGS. 9A and 9B, above the top surface 2a, which is a plane in the X-Y plane direction of the fixed substrate 2, a vibrator 5 is disposed in a state isolated from the fixed substrate 2. The vibrator 5 functions as a planar vibrating body 6. The vibrator 5 is supported via support beams 7 so as to be vibratable in the X-direction. One end side of each of the support beams 7 is fixed to the fixed substrate 2 via a fixing portion 8.
On the right and left sides (as viewed in FIG. 9A) of the vibrator 5, movable-side comb electrodes 10 (10a and 10b) are each formed outwardly in the X-direction, and fixed-side comb electrodes 11 (11a and 11b) are each disposed inwardly in the X-direction at positions opposed to the movable-side comb electrodes 10 and interdigitated therewith in a spaced relationship. The movable-side electrodes 10 and the fixed-side comb electrodes 11 are each connected to outside electrode pads (not shown) via conductive patterns (not shown), and thereby form exciting means 12.
For example, when AC voltages which are different in phase from each other by 180xc2x0, are applied to the fixed-side comb electrodes 11a and 11b while maintaining the movable-side comb electrodes 10a and 10b at a predetermined constant voltage (0 volt for example), electrostatic forces in directions opposite to each other occur between the movable-side comb electrodes 10a and the fixed-side comb electrodes 11a, and between these movable-side comb electrodes 10b and the fixed-side comb electrodes 11b, and by these electrostatic forces, the vibrator 5 is caused to be subjected to an excitation vibration in the X-direction.
In the resonant element 1 with the above-described features, when the resonant element 1 is rotated around the Y-axis while being caused to be subjected to an excitation vibration in the X-direction, as described above, a Coriolis force occurs in the Z-direction orthogonal to the X-Y plane direction. This Coriolis force is applied to the vibrator 5 (planar vibrating body 6), and the vibrator 5 vibrates in the direction of the Coriolis force. By measuring the electric signal corresponding to the magnitude of the vibration amplitude of the vibrator 5 due to the Coriolis force occurring at this time, for example, the magnitude of a rotational angular velocity can be detected.
In the case where the resonant element 1 is used as an angular velocity sensor, there is provided a detecting portion for measuring the electric signal corresponding to the magnitude of vibration amplitude of the vibrator 5 due to a Coriolis force.
When the resonant element 1 is produced, the resonance frequency of the vibrator 5 (planar vibrating body 6) in the direction of a Coriolis force (Z-direction) is previously set at the design stage to the resonance frequency in the X-direction, and the shape, dimensions, weight, etc. of the vibrator 5 are designed and implemented so that the resonance frequency is obtained. In many cases, however, the shape, dimensions, weight, etc. of the vibrator 5 are not implemented as designed because of the machining accuracy of the silicon micromachining technique. Accordingly, deviation of the resonance frequency of the vibrator 5 from the designed frequency often occurs. If the vibration of the vibrator 5 is in a resonant state, the amplitude thereof is greatly amplified by virtue of the Q (quality factor) value related to the structure, but if the frequency deviates, a problem arises in that the amplitude is not amplified nearly as much, resulting in the sensitivity of the resonant element being significantly reduced. It is therefore necessary to adjust the resonance frequency of the vibrator 5 to the set frequency in design by performing trimming with respect to the vibrator 5 and/or the support beams 7 by, for example, a complicated machining process.
The resonant element 1 is, however, a minute element, therefore, it is practically impossible, because of the accuracy of conventional mechanical trimming techniques, to perform trimming of the minute planar vibrating body 6 and/or the support beams 7 so as to have the desired dimensions, shape, and weight, etc. It has, therefore, been difficult to adjust the resonance frequency of the planar vibrating body 6 to a set value.
In the resonant element 1, therefore, a conductive layer 15 for providing an electrostatic attractive force 14 is provided on the fixed substrate 2 at the position opposed to the vibrator 5 with an interval interposed in the Z-direction, as illustrated in FIGS. 9A and 9B.
As shown in FIG. 9A, the conductive layer 15 is connected to a conductive pad 17 via a conductive pattern 16. By controlling the voltage to be applied to the conductive layer 15 via the conductive pattern 16 and conductive pad 17, the resonance frequency of the vibrator 5 is adjustable to a set value.
Once a DC voltage is applied to the conductive layer 15, an electrostatic attractive force acts on the vibrator 5, and this acts on the vibrator 5 as an electrostatic spring. Specifically, when the vibrator 5 vibrates in a direction such that the vibrator 5 approaches the fixed substrate 2, the electrostatic attractive force acts in the direction such that the amplitude is increased, so that the application of the DC voltage to the conductive layer 15 has an effect of generating a force in the direction opposite to the direction of the force of a mechanical spring. This results in a reduction in the resonance frequency of the vibrator 5 in the Z-direction. Since this reduced amount of the resonance frequency varies in accordance with the magnitude of the electrostatic attractive force 14 applied, a fine-adjustment of the resonance frequency of the vibrator 5 from the natural frequency thereof to the lower frequency side can be performed by adjusting the magnitude of the DC voltage applied to the conductive layer 15.
Utilizing this effect, by designing the natural resonance frequency of the vibrator 5 in the Z-direction to be slightly higher than the most sensitive resonance frequency (the frequency equal to the resonance frequency in the X-direction), in other words, by designing the resonance frequency of the vibrator 5 in the Z-direction to be higher than the resonance frequency thereof in the excitation vibrational direction by the exciting means 12, the vibrator 5 can be resonated at a predetermined resonance frequency by adjusting the magnitude of the DC voltage applied to the conductive layer 15.
However, although the adjustment of the resonance frequency for the vibrator 5 has been performed by providing the conductive layer 15, in some cases, characteristics such as S/N (signal-to-noise) ratio have deteriorated due to an increase in the noise of the resonant element 1.
Investigation by the present inventor into the reason for the deterioration of the characteristics, has found that the deterioration of characteristics is attributable to the vibrating conditions of the vibrator 5.
FIGS. 8A and 8B each show vibrating states of the vibrator 5 in the X-Z plane, observed in experiments by the present inventor. If there is no angular velocity around the Y-axis when the vibrator 5 is caused to be subjected to an excitation vibration, it is desirable that vibrator 5 be subjected to an excitation vibration in the X-direction horizontally along the plane 2a in the X-Y plane direction of the fixed substrate 2, as illustrated in FIG. 10C, and substantially without deflection in the Z-direction as shown in FIG. 8B.
In contrast to this, in some cases, the vibrator 5 vibrates as shown in FIGS. 10A and 10B, and as illustrated in FIG. 8A, the vibrations of the vibrator 5 are in states deflecting by a large amount in the Z-direction which is the detection direction of a Coriolis force. In such cases, the resonant element 1 has been found to deteriorate in characteristics.
The present inventor, therefore, has noted that the deflection of the vibrator 5 in the Z-direction is attributable to the tilt of the vibrator 5 with respect to the substrate plane of the fixed substrate 2, and has proposed various resonant elements 1 each having excitation deflection inhibiting means for correcting the tilt of the vibrator 5 and for inhibiting the deflection of the vibrator 5 in the Z-direction.
FIG. 6A is a perspective view showing one example of a previously proposed resonant element 1, and FIG. 6B is a sectional view taken along the line Ixe2x80x94I in FIG. 6A. Here, in FIGS. 6A and 6B, the same components as those of the resonant element 1 in FIGS. 9A and B are identified by the same reference numerals, and repeated descriptions of the common components are omitted. It is to be noted that the resonant element 1 shown in FIGS. 6A and 6B is disclosed in Japanese patent application Nos. 11-197096 and 11-344648 and the corresponding U.S. and European patent applications.
In the resonant element 1 shown in FIGS. 6A and 6B, conductive layers 20 and 21 are disposed on the plane in the X-Y plane direction of the fixed substrate 2 so as to be opposed to each other with a gap in the X-direction therebetween, and to be opposed to the right or left (as viewed in FIG. 6A) edge area with a gap interposed. The conductive layers 20 and 21 are conductively connected to conductive pads 24 and 25 via conductive patterns 22 and 23, respectively. In the resonant element 1 shown in FIG. 6A and 6B, the conductor layers 20 and 21 constitute excitation deflection inhibiting means.
By individually applying DC voltages to the conductive layers 20 and 21 via the conductive patterns 22 and 23, and the conductive pads 24 and 25, respectively, electrostatic attractive forces 26 and 27 occur between the conductive layers 20 and 21, and the vibrator 5. The tilt of the vibrator 5 can be corrected by adjusting each of the right and left electrostatic attractive forces 26 and 27 to the vibrator 5 through adjusting each of the voltages to be applied to the conductive layers 20 and 21.
Specifically, if the vibrator 5 tilts downwardly to the right as indicated by a broken line xcex1 in FIG. 6B, a DC voltage higher than that applied to the conductive layer 21 is applied to the conductive layer 20. Thereby, the electrostatic attractive force 26 acting on the left edge area of the vibrator 5 opposed to the conductive layer 20 becomes larger than the electrostatic attractive force 27 acting on the right edge area of the vibrator 5 opposed to the conductive layer 21, so that the left edge area of the vibrator 5 is pulled toward the fixed substrate 2 side more strongly than the right edge area of the vibrator 5, whereby the downward tilt to the right is corrected.
If the vibrator 5 tilts upwardly to the right as indicated by a broken line xcex2 in FIG. 6B, a DC voltage higher than that applied to the conductive layer 20 is applied to the conductive layer 21. Thereby, the electrostatic force 27 becomes larger than the electrostatic force 26, so that the right edge area of the vibrator 5 is pulled toward the fixed substrate 2 side more strongly than the left edge area of the vibrator 5, whereby the downward tilt to the right is corrected.
In the resonant element 1 shown in FIG. 6, by providing the conductive layers 20 and 21, the tilt of the vibrator 5 can be corrected, which leads to an improvement in the vibrating conditions of the vibrator 5.
Meanwhile, in order to find the optimum applied voltage to the conductive layers 20 and 21 for correcting the tilt of the vibrator 5, the present applicant has performed the following vibration adjustments for the vibrator 5, utilizing a large-scale vibration measuring system 30 as shown in FIG. 7.
For example, a resonant element 1 is disposed on a specimen holding stand 31, driving means (not shown) for AC voltage application is conductively connected to the fixed side comb electrodes 11, and first DC bias applying means 32 is conductively connected to the conductive layer 20 via the electrode pad 24 and the conductive pattern 22. In the same manner, second DC bias applying means 33 is conductively connected to the conductive layer 20 via the electrode pad 25 and the conductive pattern 23.
Then, in the state wherein the vibrator 5 is caused to be subjected to an excitation vibration by the driving means, the vibrator 5 under excitation vibration is radiated by laser rays 35 from a laser displacement meter 34. By utilizing the reflected laser rays from the vibrator 5, signals in response to displacements in the X-direction and in the Z-direction of the vibrator 5 are outputted from the laser displacement meter 34 to an oscilloscope 36.
The vibrating conditions of the vibrator 5 in the X-Z plane can be viewed on the screen of the oscilloscope 36. While viewing the screen of the oscilloscope 36, a vibration adjustment operator individually varies the magnitudes of the voltages to be applied to the conductive layers 20 and 21, by controlling the first and second DC bias applying means 32 and 33, respectively. Thereby, the vibration adjustment operator obtains the optimum values of the applied voltage to the conductive layers 20 and 21, wherein the deflection of the vibrator 5 in the Z-direction can be eliminated, or suppressed to a very small amount.
In this manner, vibration adjustments for the vibrator 5 utilizing the vibration measuring system 30 shown in FIG. 7 are performed.
After the optimum values of the applied voltage to the conductive layers 20 and 21 have been obtained as described above, the resonant element 1 is removed from the specimen holding stand 31 of the vibration measuring system 30, and then, for example, is built into a predetermined sensor device or the like. The sensor device into which the resonant element 1 having the conductive layers 20 and 21 shown in FIGS. 6A and 6B is to be built, is provided with means for individually applying DC voltages to the conductive layers 20 and 21, and the means is set so that voltages having the optimum values are each applied to the conductive layers 20 and 21. This allows the vibrator 5 of the resonant element 1 to be subjected to an ideal excitation vibration in the X-direction without deflection in the Z-direction, and allows the resonant element 1 to be improved in characteristics.
However, there has been a problem that, in order to obtain the above-described optimum values of the applied voltages to the conductive layers 20 and 21, a large-scale and costly vibration measuring system 30 is required. Also, as described above, the vibration adjusting method for the vibrator 5 has been a manual method such that the operator obtains the optimum values of the applied voltages to the conductive layers 20 and 21 by controlling the first and second DC bias applying means 32 and 33, respectively, while viewing the screen of the oscilloscope 36. As a result, the vibration adjustment is both costly and time consuming.
Furthermore, as described above, since the operator performs the vibration adjustment for the vibrator 5 based on vibration conditions (vibrational loci) of the vibrator 5 in the X-Z plane while viewing the screen of the oscilloscope 36, there is a problem in that an improvement in the adjustment accuracy is limited by the skill of the operator.
Moreover, in accordance with the vibration adjustment method as described above, after the optimum value of the applied voltage to the conductive layers 20 and 21 has been obtained by the vibration measuring system 30, the resonant element 1 is removed from the specimen holding stand 31 of the vibration measuring system 30, and is built into a predetermined sensor device or the like. Therefore, even though the optimum values of the applied voltages to the conductive layers 20 and 21 have been obtained by the vibration measuring system 30, for example, the stresses within the support beams 7 of the resonant element 1 can change when the resonant element 1 is removed from the specimen holding stand 31 and is built into a predetermined sensor device, and thereby the optimum voltage values of the applied voltages to the conductive layers 20 and 21 can change into different voltages from the optimum voltages values. In such a case, the vibrator 5 of the resonant element 1 in the sensor device will not be subject to an optimum excitation vibration.
If the optimum voltage value of the applied voltage to the conductive layers 20 and 21 has thus changed, it will be necessary to again dispose the resonant element 1 in the vibration measuring system 30 and to again perform the above-described vibration adjustment. This is very troublesome, however, and it is virtually impracticable after the resonant element 1 has been built into the sensor device.
The present invention solves the above-described problems. It is a first object of the present invention to allow the vibration adjustment for the vibrator for inhibiting the deflection of the vibrator (vibrating body) in the Z-direction during the excitation vibration thereof in the X-direction to be easily performed, without the need for a large-scale apparatus. It is a second object to facilitate the automation of the vibration adjustment for the vibrator. It is a third object to provide a resonant element allowing the vibration adjustment for the vibrator to be performed in the state wherein the vibrator has been built into the sensor device.
In order to achieve the objects, the present invention has the following constitutions as means for solving the above-described problems. In accordance with a first aspect, the resonant element includes a vibrating body vibratable in orthogonal X- and Z-directions; exciting means for causing the vibrating body to be subjected to an excitation vibration in the X-direction; excitation deflection detecting means for detecting the deflection of the vibrating body in the Z-direction during the excitation vibration thereof in the X-direction; and excitation deflection inhibiting means for inhibiting the deflection of the vibrating body in the Z-direction.
The resonant element in accordance with another aspect constitutes an angular velocity sensor for detecting the angular velocity around a Y-axis orthogonal to the X- and Z-direction based on the vibration of the vibrating body in the Z-direction by a Coriolis force, and the angular velocity sensor has Z-direction vibration detecting means for detecting the vibration of the vibrating body in the Z-direction, the Z-direction vibration detecting means also serving as excitation deflection detecting means.
In accordance with another aspect, the excitation deflection detecting means is constituted of a detecting electrode for detecting the variation in the electrostatic capacity with respect to the vibrating body in response to the vibration of the vibrating body in the Z-direction, and the excitation deflection detecting means detects the variation in the detected electrostatic capacity by the detecting electrode during the excitation vibration thereof in the X-direction, as a deflection of the vibrating body in the Z-direction.
In accordance with another aspect, the vibrating body is disposed so as to be opposed to the plane of the X-Y plane direction of the fixed substrate, and the vibrating body constitutes a planar vibrating body supported by the fixed substrate via support beams so as to be vibratable in the X-direction.
In accordance with another aspect, the invention is directed to a vibration adjustment method for a resonant element which includes a vibrating body vibratable in orthogonal X- and Z-directions, exciting means for causing the vibrating body to be subjected to an excitation vibration in the X-direction, a detecting electrode for detecting the variation in the electrostatic capacity with respect to the vibrating body in response to the vibration thereof in the Z-direction; and excitation deflection inhibiting means which give electrostatic attractive forces to the vibrating body and which inhibit the deflection of the vibrating body in the Z-direction during the excitation vibration thereof in the X-direction by the electrostatic attractive forces. In the vibration adjustment method in accordance with this aspect, the variation in the detected electrostatic capacity by the detecting electrode is detected as a deflection of the vibrating body in the Z-direction, while the vibrating body is caused to be subjected to an excitation vibration in the X-direction by the exciting means; and the electrostatic attractive forces given to the vibrating body by the excitation deflection inhibiting means are controlled in the direction such that the variation in the detected electrostatic capacity by the detecting electrode is canceled.
The vibration adjustment method for a resonant element in accordance with another aspect converts the detected electrostatic capacity by the detecting electrode into a voltage, the deflection of the vibrating body in the Z-direction during the excitation vibration thereof in the X-direction is detected based on the variation in the voltage.
The vibration adjustment method for a resonant element in accordance with another aspect, converts the detected electrostatic capacity by the detecting electrode into a voltage using capacity-voltage converting means comprising FET.
In a vibration adjustment method for a resonant element in accordance with another aspect, the resonant element constitutes an angular velocity sensor for detecting the angular velocity around the Y-axis by a Coriolis force based on the vibration of the vibrating body in the Z-direction. The angular velocity sensor has Z-direction vibration detecting means for detecting the vibration of the vibrating body in the Z-direction utilizing the variation in the electrostatic capacity; and capacity-voltage converting means for converting the detected electrostatic capacity by the detecting electrode into a voltage, the Z-direction vibration detecting means also serving as excitation deflection detecting means; and the capacity-voltage converting means also serves as capacity-voltage converting means for detecting an excitation deflection.
In the present invention, when performing vibration adjustment for the resonant element by providing the resonant element with excitation deflection detecting means and excitation deflection inhibiting means, the deflection of the vibrating body in the Z-direction during the excitation vibration thereof in the X-direction is detected by the excitation deflection detecting means, and the excitation deflection inhibiting means are adjusted in a direction such that the deflection in the Z-direction is eliminated. Thereby, the deflection of the vibrating body in the Z-direction during the excitation vibration thereof in the X-direction can be inhibited, and the vibrating body can be caused to be subjected to an ideal excitation vibration in the X-direction. This makes it possible to avoid the problem of the characteristics deterioration of the resonant element caused by the deflection of the vibrating body in the Z-direction.
Since the resonant element is in itself provided with the excitation deflection detecting means, there is no need for large-scale equipment for measuring the vibrating conditions of the vibrating body as described above, which leads to a reduction in equipment cost.
Also, since the vibration adjustment for the vibrator of the resonant element can be performed in the state wherein the vibrator has been built in the sensor device, it is possible to prevent the problem that, even though the vibration adjustment for the vibrating body has been performed stresses, for example, within the support beams supporting the vibrating body change when the resonant element is built into the sensor device, resulting in the vibrating body not being subjected to an ideal excitation vibration.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.